High-quality meta-analyses, systematic reviews of RCTs or RCTs with a very low risk of bias

1+

Well-conducted meta-analyses, systematic reviews or RCTs with a low risk of bias

1

Meta-analyses, systematic reviews or RCTs with a high risk of bias

2++

High-quality systematic reviews of case–control or cohort studiesHigh-quality case–control or cohort studies with a very low risk of confounding or bias and a high probability that the relationship is causal

2+

Well-conducted case–control or cohort studies with a low risk of confounding or bias and a moderate probability that the relationship is causal

2−

Case–control or cohort studies with a high risk of confounding or bias and a significant risk that the relationship is not causal